Veatchite: Structural relationships of the three polytypes

Abstract

Crystal structures of the three polytypes of veatchite, Sr₂B₁₁O₁₆(OH)₅·H₂O, are determined by X-ray, single-crystal studies. The polytypes are: veatchite-1A, triclinic space group P1̄, with a = 6.6378(6), b = 6.7387(6), c = 20.982(2) Å, α = 87.860(1), β = 82.696(12), γ = 60.476(1)°, V = 809.7(2) ų; veatchite-1M, monoclinic space group P2₁, with a = 6.7127(4), b = 20.704(1), c = 6.6272(4) Å, β = 119.209(1)°, V = 805.4(2) ų; and veatchite-2M,monoclinic space group Cc with a = 6.6070(3), b = 11.7125(5), c = 20.6848(9) Å, β = 91.998(1)°, V = 1599.7(2) ų. The crystal structures have two layer types with similar fundamental building blocks: A layer FBB = 3Δ2□:<Δ2□>-<2Δ□> and B layer FBB = 3Δ2□,1Δ:<Δ2□>-<2Δ□>,Δ (Grice et al. 1999). Unique in this FBB is the lone polyhedron with triangular coordination, which consists of a neutrally charged [B(OH)₃] group. Layering has a directional component and depending on layer sequence the symmetry may be centrosymmetric or non-centrosymmetric. Related layered borate structures, biringuccite, nasinite, gowerite, and volkovskite, are compared. Observations indicate that veatchite-1A is the low-temperature and low-pressure polytype, veatchite-1M the high-temperature polytype, and veatchite-2M the moderate temperature and higher pressure polytype.